This invention relates in general to xeroradiographic imaging and, in particular, to a new improvement in xeroradiographic imaging.
The xeroradiography relates to the recording of X-ray patterns and information by means of materials and devices whose electrical conductivity is altered by the action of penetrating radiation, such as X-rays and the like. In the xeroradiographic process, a normally insulating X-ray sensitive layer overlaying a conductive backing surface may be charged electrically and used as a radiation sensitive recording surface because of conductivity imparted by the radiation. Thus, the appropriate surface, such as, for example, a conductive metallic surface having a vitreous or amorphous selenium photoconductive layer, may be charged and exposed to an X-ray pattern, as disclosed in Schaffert U.S. Pat. No. 2,666,144. A latent electrostatic image is thereby provided or a xeroradiographic latent image which may be developed by dusting with finely divided charged particulate material referred to in the art as toner to form a visible image.
Normally, the electrostatic latent image is developed by presenting the plate bearing the image to a cloud or gas suspension of charged powder particles or toner. The image as developed in this way, see Jeromin et al U.S. Pat. No. 3,640,246, has a remote grid with a backing bias to distinguish the polarity of toner desired to be used in the development of the image. This remote electrode developed image is characterized by the emphasis of the development of electrostatic fringe fields created by discontinuities between charged and lower charged areas. These developed images are distinguished by a "halo" of undeveloped or lightly developed region around these different potential areas. The size and degree of this "halo" is a function of the degree of potential difference and of the sharpness of the discontinuity. In a true fringe field developed image there is no developed density difference between the central regions of each potential area. The image is recognized purely by the "halo" which results in a pseudodensity difference, well known as the Cornsweet Effect. When a very close spaced electrode is used an absolute development occurs. The toner deposited on the plate is a function of the charge potential and there is no fringe effect. In actual practice, neither of these exist in true form and a combination of these exists in the Jeromin et al Patent and co-pending Klingenberg and Klingenberg et al applications, Ser. Nos. 525,698, filed Nov. 21, 1974, and 525,697, filed Nov. 21, 1974, now U.S. Pat. No. 3,974,796, respectively which are hereby incorporated by reference. The former patent is characterized by emphasis on fringe field development, and in the latter applications, the tendency is toward absolute development. The apparatus disclosed in Klingenberg applications may be utilized to provide both the far spaced and close spaced electroded modes since it provides a movable grid.
The halo property however, which is the very property which produces these preferential results, produces an additional and in some cases an undesirable result. Very frequently in X-ray examination it is desired, for example, in examining industrial castings or the like, to locate casting flaws or, in medical examination to locate and analyze tissue and bone areas when the area of particular interest in the X-ray examination is an area at or near a point of sharp contrast differential. Thus, in medical examination, it may be desired to detect a bone injury at or near a joint or, in an industrial examination, it may be desirable to detect flaws which might be more prevalent near seams, joints, or sharp contour changes in the test object. For example, if a casting is being analyzed by X-ray examination, the purpose of the examination very often is to detect the presence or absence of flaws associated with sharp angles in the surfaces. It is exactly this type of flaw that could occur in the halo area of the xeroradiographic plate and which is somewhat more likely to be partially missed by the halo itself.
In medical examinations, and more specifically, in xeromammography applications, it is of utmost importance that edge enhancement be present but in addition it may be desirable that areas adjacent to areas having high edge enhancement be made readily visible to the investigator. This capability may be extremely desirable to the medical technician in certain investigations. In addition, it is also desirable in such examinations to detect soft tumors or growths or structures which do not have sharply defined edges or slowly changing boundaries.
It is therefore an object of this invention to provide a xeroradiographic imaging system devoid of the above-noted deficiencies.
It is another object of this invention to provide a novel system for xeroradiographic examination having increased readability of the X-ray record in areas of sharply varying contrast.
Still another object of this invention is to provide a novel xeroradiographic system having enhanced contrast sensitivity with particular reference to contrast sensitivity in areas of sharp contrast gradation.
Again another object of this invention is to provide a xeroradiographic imaging system which possesses edge enhancement in addition to the ability to image slowly changing boundaries.
Yet another object of this invention is to provide a novel xeromammographic imaging system edge providing enhancement of calcifications as well as imaging slowly changing borders such as found in soft tumors.
These and other objects of the instant invention are accomplished, generally speaking, by providing a xeroradiographic system which employs both fringe field and absolute development. This is accomplished by first developing the image with a colored toner in the fringe field, i.e., far spaced electroded mode, and then developing the electrostatic image in the absolute mode, i.e., employing a closely spaced electrode to the latent electrostatic image with a constrasting colored toner.
A xeroradiographic plate, for example, a layer of photoconductive selenium disposed over a conductive aluminum substrate, is charged and then exposed to penetrating radiation which passes through a test object to be imaged resulting in the formation of a xeroradiographic latent image. This image is then developed employing a conventional powder cloud development type system with the exception that a far spaced electrode is employed allowing development in the fringe field to provide edge enhancement. In this first development broad area coverage is suppressed and edge enhancement is emphasized. Development of a fringe field results in edge enhancement and produces a halo effect. The plate is then subjected again to the same powder cloud development but during this step the system is electroded so that absolute development is provided with the use of an electrode closely spaced to the xeroradiographic plate to provide broad area coverage and suppress edge enhancement. Broad area coverage is accomplished in the close spaced electrode mode, i.e., absolute development because the closely spaced electrode causes the otherwise wandering field to orient and stand so that when toner is pulsed onto the plate by employing a conventional powder cloud system development is effected directly in relation to the charge remaining on the surface of the xeroradiographic plate since the field is made to stand substantially perpendicularly from the xeroradiographic plate.
In U.S. Pat. No. 2,879,397, having a common assignee and the identical inventor, a similar but distinguishable system is provided wherein development of the halo areas is accomplished. In the Lehmann patent, a dual development procedure is employed in which a xeroradiographic latent image is first developed under conditions enhancing detail presentation in certain areas of the image, and the image is subsequently developed under different conditions of opposite charge, enhancing detail presentation in the remaining areas of the image. According to Lehmann, this is accomplished by successive development steps in one of which the development is carried out with a cloud or gas suspension of finely divided particles of one color and electrical polarity followed by a second development step with a cloud or gas suspension of particles of a contrasting color in opposite electrical polarity. This dual operation results in particles of one color preferentially deposited or developed in relatively large areas and areas of gradual contrast differential, whereas particles of the contrasting color are preferentially developed or deposited in what are normally the halo areas of a xeroradiographic print. Thus, Lehmann produces a xeroradiographic print retaining essentially the advantages of enhanced readability produced by the halo characteristic while at the same time substantially overcoming the masking effect of halo with respect to sharp contrast areas. However, Lehmann employs a distinctly different process which does not employ a far spaced and close spaced electrode and therefore does not obtain both edge enhancement and broad area coverage which allows detection of slowly changing boundaries as does the instant invention. The process of the instant invention, therefore, constitutes a substantial advance over the Lehmann xeroradiographic system.
In Hukase et al, U.S. Pat. No. 3,854,043, an electrostatic latent image formed on a photoconductive layer by means of X-ray electrophotography is divided into at least two regions by the degree of electrostatic potential, said regions being represented by different colors by depositing a different color developer powder on each of said regions. Thus, Hukase et al employs biased development with one colored toner of a given polarity and then bias develops again at a different level with a contrasting colored toner of the same charge. Through this distinguishable process, Hukase et al allows development of different thicknesses with different colors but does not provide both edge enhancement and the ability to image slowly changing boundaries by employing a far spaced and close spaced electrode as in the system of the instant invention.